It is customary to employ a pump to move and deliver liquid, and to vary the pressure thereof. If a higher pressure is required, two or more pumps are employed as required. Also, in almost all devices designed to handle liquid, leakage of the liquid from the devices is a problem in the design, operation and maintenance of such devices.
It is obviously more difficult to design such a device when the pressure and temperature of the liquid are high or the liquid is dangerous or harmful. In order to raise the pressure, one or more additional pumps may be installed. For example, in a pumping system handling chemical solution under high pressure and at high temperature, associated equipment such as a liquid or solution reservoir, absorption tower and/or reaction tank may be provided in double and two pumps will be employed so as to deliver the solution sequentially. If two pumps are employed, it is necessary to provide a shaft sealing mechanism at four places. Also, it is more difficult to seal the second pump effectively, due to the higher pressure and the higher temperature of the solution in the second pump. Should the solution be dangerous or harmful, failure in the seals would cause serious problems. Thus, those factors involved in the installation of plural pumps in series result in a substantial increase in the cost of the whole system.
One attempt to solve this problem is to combine the two pumps into a single unit comprising two pump sections and to transfer the solution outside once from the intermediate portion between the two sections to the second stage of the unit through the first reservoir, absorption tower or reaction tank thereby reducing the total number of sealing portions required to two. In such multi-stage pump, the axial thrust is usually balanced axially with respect to the whole unit. However, in such multi-stage pump, the balance of the axial thrust may not be maintained and the resulting thrust may become very large, if the operating conditions or the requirements in each of the both sections are varied.
It is also well known that a certain operational range for a minimum flow rate is usually specified for a certain pump and such range becomes narrower as the temperature and pressure of the liquid to be pumped become higher. In the operation of a pump at minimum flow rate, the temperature of the liquid passing the pump becomes relatively high compared to that under the operation of the pump at nominal or ordinary flow rate. This increases the possibility of failure at the sealing portions, which may result in leakage or evaporation of the liquid. Also, in the composite multi-stage pump including plural pump sections, the general tendency is that the temperature of the liquid or solution, especially if chemical reactions are involved, is raised as the liquid or solution is fed from the lower pressure side to the higher pressure side. Under such condition, should the liquid or solution of higher temperature be introduced to the side of lower pressure and the lower temperature, vaporization would occur within the pump section which might put the pump impeller in unoperable condition.